Effect of Pressure on the Deuteron Spin-Lattice Relaxation Times and the Self-Diffusion Coefficient in Ionic Liquids

Ionic liquids (ILs) are of considerable interest for their physical properties: negligible vapor pressure, thermal and electrochemical stability, and high ionic conductivity, thus making them attractive for applications in radioactive material handling, batteries, capacitors, and electrochemical solar cells. A large variation of physical properties of ILs results from a corresponding variation of the structure and length of the cation sidechains and choice of anion, thus allowing tunability of properties for selected applications. In this context, there is much interest in the molecular motions that control transport properties and chemical processes. In this work we present investigations of a set of ILs based on the EMIM and BMIM cations and TFSI anion, in which the cation sidechains were partially deuterated. The ²H spin-lattice relaxation times (T₁) have been measured at 47 MHz in the temperature range from 240 to 370 K (Graph 1) and under hydrostatic pressure from ambient to 250 Mpa. (Graph 2) with the deuteron T₁ providing a particularly sensitive probe of alkyl side-chain reorientation. Additionally the self-diffusion coefficient (D) of both cation (using 1H NMR) and anion (using 19F NMR) were conducted as a function of temperature and Pressure (Graph 3). The Arrhenius plots of T₁ exhibit T₁ minima, and analyzed assuming an exponential decaying correlation function (BPP model). The high pressure T₁ and D measurements were carried out to determine the activation volume of alkyl chain motion.  Longer alkyl chain of BMIM exhibits a pressure invariance of the relaxation time after 80 Mpa possibly related to interactions between alkyl groups on adjacent cations. Hunter College acknowledges the U.S. Office of Naval Research for supporting the NMR portion of this project. The work at BNL was supported by the U. S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences under contract # DE-AC02-98CH10886.